All-optical fiber-optic lines require light signal transmission along the line without its conversion into electrical one and vise versa. For this purpose a number of all-optical switches based on different technologies were introduced.
There are a few technologies of all-optical switching utilized now in fiber-optic telecommunication lines, such as:                MEMS switching technology,        Acousto-optic switching technology,        Mechanical switching technology.        
To connect fiber-optic channels, MEMS switches utilize electrically-controlled movable micro-mirrors. Acousto-optic switches utilize deflecting property of acoustic wave running in special crystals, such as TeO2 ones. Mechanical switches utilize movable optic fibers to connect fiber-optic channels. There are some attempts to utilize other physical effects for this purpose, such as electro-optics, micro-bubble, etc., but switches based on these effects still under development and do not reveal characteristics suitable for fiber-optic lines.
The switching technologies mentioned above have its individual pattern of application. For example, MEMS switch can provide multi-channel (up to hundred channels) N×N cross-connection and have switching time of 1 millisecond and more. Acousto-optic switches provide multi-channel (up to thousand channels) 1×N and N×1 connections with switching time of 3-5 microseconds.
Mechanical switches can connect a few fiber-optic channels at low speed, such as 1 second and more. Therefore, the mechanical switches can be successfully utilized where high-speed-switching is not required, for example, in multi-channel fiber-optical laboratory measuring devices and in fiber-optic telecommunication lines as switches for channel regeneration.
Existed mechanical fiber-optic switches use mechanical movement of open-end optic fiber, which, being controlled by precession mechanical actuator is going to close proximity with another optic fiber in such a way that allows transmitting light through a tiny gap between the fibers.
Such solution has obvious disadvantages:
The actuator has to have accuracy of a couple microns; otherwise optical losses will be very high. The switching time of such mechanical switches is around of a few seconds. Because of this, such switches can find limited applications. Also, all mentioned fiber-optic switches including MEMS and acousto-optic ones have open space, where the spatial switching is performed. So, all elements of such switches have to have enough strength to keep position of elements with preciseness of a couple microns. Also, all mentioned switches are electronic-controlled ones, so it requires special electronic drivers that do not allow utilizing them in fire or explosive-hazardous environment.
The object of the present invention—a mechanical fiber-optic switch—utilizes phenomenon of increasing of light attenuation induced in single-mode optic fiber under its bending.
In these fibers, the single-mode light propagation is based on light diffraction and the bending changes diffraction-conditions so introducing additional light attenuation.
The experiments conducted by the author of the present invention reveal following features of a single-mode fiber:                The light attenuation depends on the fiber bending radius and arc angle of the bending.        If bending radius of a length of single-mode optic fiber becomes smaller than about 9 mm, the intensity of a single-mode light running in the fiber gradually declines.        When the bending radius becomes smaller than two millimeters, the light transmission is completely terminated. This effect appears in a conventional 9/125-micrometer single-mode optical fiber.        
These experiments show that a single-mode light attenuation introduced by the bending of a single-mode fiber can be described by formula:Pout/Pin=[f(r)]φ  (1),where f(r)—the function of light attenuation from radius of bending, φ—arc angle of bending (in radians), Pin—input power of the light and Pout—output power. Because in telecommunication industry signal amplification and attenuation is measured in decibels (A [dB]=10 log Pout/Pin), the formula above can be transformed into the logarithmic one. Thus, this formula (for fixed-radius arc) looks as:A [dB]=F(r)φ  (2),where F(r)=log f(r) and φ—arc angle of bending (in radians).
For variable radius this formula looks as:δA [dB]=F(r)δφ  (3).
Here, F(r) can be defined as the specific attenuation [dB/rad].
Because freely bent silica fiber has complicated shape, not a circular one, F(r) varies along the fiber, and this formula looks as:A [dB]=Σ(δAi)=ΣF(ri)δφ  (4).
The function of specific attenuation F(r) (in dB/rad) from radius of bending (r) taken at 1310-nm wavelength is shown on FIG. 1. It is a non-linear one that significantly increases for lesser radius and asymptotically rises at 2.5-mm radius. This dependence in the first approximation can be described by the formula (for 9/125 single mode fiber, 1310-nm wavelength and radius range from 3.5 mm to 8 mm):F(r)=(4.4/r)4.6  (5),where F(r) is taken in dB/rad, and r—in mm.
The total attenuation A linearly depends on the angle of bending. It means that attenuation (measured in dB) is twice higher for 360-degree loop than for 180-degree arc. For example, attenuation measured for 9.6-mm bending diameter and 180-degree arc is 2.2 dB at 1310-nm wavelength, and for 360-degree loop of the same diameter is 4.4 dB. When the fiber is coiled as a multi-turn winding, the attenuation (in dB) increases proportionally to the number of turns. Thus, the formula for total light attenuation (1310-nm wavelength) induced in the multi-turn winding can be described by the formula:A [dB]=2πN(4.4/r)4.6  (6),where radius r is taken in mm, and N—the number of turns.
Therefore, utilizing variable-shape multi-turn coil of optic fiber it becomes possible to use higher radii of the bending, such as 6-7 mm. In this case, because the range of working displacement becomes larger, mechanical fatigue of the fiber and probability of its failure appearing after number of bending become much smaller and does not decline switch lifetime.
Basing on mentioned above experiments, the method of the fiber bending and a bending device were developed and implemented in fiber-optic switch—the object of the present invention.
The research conducted by the author of the present invention also reveals that there are a couple of principles of fiber bending that was further utilized in the present invention, which can allow creating the fiber-optic switch with stable parameters. One of these principles is to avoid sharp bending. Such bending produces high attenuation, but it is unstable, affected by small unwanted displacements and can cause the fiber failure. Also, to avoid unwanted bending of intermediate parts of the optic fiber, the fiber ends has to be tangential to the fiber loop. For example, for circular (or elliptical) multi-turn winding, the end fibers have to be tangential to the winding. In the case of bending around a fixed radius shaft, the end fibers, also, has to be tangential to the shaft circle. Those principles are described by the drawings on FIG. 2.
FIG. 2 A depicts the bending method that, in particular, was utilized in fiber-optic gage described in U.S. Pat. No. 5,818,982 issued Oct. 6, 1998 to Voss. Here, freely bent fiber has a complicated Ω shape with sharp bent parts. These parts provides high attenuation, but they are unstable, affected by small unwanted displacements and can cause the fiber failure.
To solve this problem, the transformation of multi-turn circular winding into elliptical one was proposed in a number of patents and patent pending, such as U.S. Pat. No. 5,050,946 issued to Sorensen, Us Patent applications No 2004/0047583 and 2005/0047745. Here authors proposed multi-turn windings of optical fiber that freely coiled between two rods in such a way that, when the rods are shifted, the coil is stretched into elliptical one so introduce high attenuation of optical signal running in the coil. Therefore, such attenuation completely closes the optical circuit where the coil is installed.
Experiments conducted by the author of the present invention reveal that freely coiled between two rods is not stable because the plane of the winding moves in shifting sequences (if the rods does not have special groves for each individual turn of fiber). The natural flexure of optic fiber utilized in such design, unlike specially-designed metal spring, is not stable and, anyway, is affected by fatigue.
Therefore, this design has to be improved by utilization of spring mandrel fixed between the rods, wherein the optical fiber is reeled and fixed on the cylindrical surface of the mandrel. Initially, the mandrel has circular shape; and it is transformed into elliptical one when the rods stretch the mandrel. Thus, all mechanical stresses are applied to the mandrel, not to optical fiber. This solution was utilized by the author of the present invention in the fiber-optical gage described in U.S. patent application Ser. No. 11/163,917. Also, the end fibers connecting the winding with fiber-optical connectors are tangentially positioned to the fiber curve. So, the end fiber is not bended when the winding is stretched. FIG. 2 C depicts transformation of multi-turn winding from circular into elliptical one. [That technology was utilized in fiber-optic gages described in U.S. patent application Ser. No. 11/163,917 filed by the author of the present invention and in fiber-optic switches of the present invention.]
Here, such transformation is performed by radial stretching of the winding. In this case, the curvature of the fiber reeled on the mandrel is changed together with the curvature of the mandrel according to the mathematics formulas derived by the author of the present invention; it is predictable and stable. Also, the end fibers are tangentially positioned to the fiber curve. The light attenuation produced by such bending can be calculated by formulas (4) and (5). Therefore, this solution allows creating on-off fiber-optic switches. These switches, unlike to mentioned above conventional fiber-optic switches, do not have any air gaps, do not require precession actuator and can be powered by any mechanical movement. Such advantages allow utilizing not electrical actuators, such as pneumatic ones. Therefore, these switches can work in variety of ambient conditions including underwater and, also, fire and explosive-hazardous ones. The switches together with fiber-optic gages described in U.S. patent application Ser. No. 11/163,917 can be utilized in all-optical fiber-optic data acquisition systems working in such hazardous facilities as oil refinery, oil wells, gas pipelines, chemical factories, munitions deport, etc. In these cases the switches can be powered by compressed nitrogen, for example. Such data collecting system can perform programmable monitoring of remote objects gathering information from large number of the optic gages.
For multi-channel switching, these on-off switches are connected to fiber-optic splitter/combiner. Such combination allows switching single-mode fiber-optical lines in time sequences. These data acquisition system allows utilizing regular single-mode fiber-optical lines, similar to ones used for telecommunication. It can be “dark fibers” or any fiber-optical telecommunication lines modified for transmission of analog signal.